Base station, communication terminal, transmission method, and reception method

ABSTRACT

A base station includes a scheduler configured to perform frequency scheduling for each subframe; a control channel generating unit configured to generate a control channel including common control information to be mapped to radio resources distributed across a system frequency band and specific control information to be mapped to one or more resource blocks allocated to each selected user device; and a transmission signal generating unit configured to generate a transmission signal by time-division-multiplexing the common control information and the specific control information according to scheduling information from the scheduler. The common control information includes a format indicator representing one of preset options that indicates the number of symbols occupied by the common control information in one subframe. The common control information includes information units with a predetermined data size. The number of the information units is less than or equal to a specified multiplicity included in broadcast information.

TECHNICAL FIELD

The present invention generally relates to wireless communicationtechnologies. More particularly, the present invention relates to a basestation, a communication terminal, a transmission method, and areception method used in a communication system where frequencyscheduling and multicarrier transmission are employed.

BACKGROUND ART

In the field of wireless communication, there is a growing demand for abroadband wireless access system that enables efficient, high-speed,high-volume communications. For downlink in such a system, amulticarrier scheme such as orthogonal frequency division multiplexing(OFDM) is expected to be used to achieve high-speed, high-volumecommunications while effectively restraining multipath fading. Also, innext generation systems, use of frequency scheduling is proposed toimprove the frequency efficiency and thereby to increase the throughput.

As shown in FIG. 1, in next generation systems, a system frequency bandis divided into multiple resource blocks (in this example, threeresource blocks) each including one or more subcarriers. The resourceblocks may also be called frequency chunks. Each terminal is allocatedone or more resource blocks. In a frequency scheduling method, toimprove the transmission efficiency or the throughput of the entiresystem, resource blocks are allocated preferentially to terminals withgood channel conditions according to received signal quality or channelquality indicators (CQIs) measured based on downlink pilot channels andreported by the terminals for the respective resource blocks. A pilotchannel is a signal known to both the sending end and the receiving end,and may also be called a reference signal, a known signal, and atraining signal. When frequency scheduling is employed, it is necessaryto provide the terminals with scheduling information indicating theresults of scheduling. The scheduling information is reported to theterminals via control channels. A control channel may also be called anL1/L2 control signaling channel, an associated control channel, or aphysical downlink control channel (PDCCH). The control channel is alsoused to report a modulation scheme (e.g., QPSK, 16 QAM, or 64 QAM) andchannel coding information (e.g., channel coding rate) used for thescheduled resource blocks as well as information regarding hybridautomatic repeat request (HARQ). For the structure of control channelsused in such a mobile communication system, see, for example, 3GPP,TR25.848, “Physical layer aspects of UTRA High Speed Downlink PacketAccess” and 3GPP, TR25.896, “Feasibility study of enhanced uplink forUTRA FDD”.

Here, when a resource block common to all terminals is staticallyallocated for a control channel, some terminals cannot receive thecontrol channel with good quality because channel conditions of aresource block differ from terminal to terminal. Meanwhile, distributinga control channel to all resource blocks may make it possible for allterminals to receive the control channel with certain reception quality.However, with this method, it is difficult to further improve thereception quality. For these reasons, there is a demand for a method oftransmitting a control channel with higher quality to terminals.

In a system where adaptive modulation and coding (AMC) is employed,i.e., where the modulation scheme and the channel coding rate areadaptively changed, the number of symbols used to transmit a controlchannel varies from terminal to terminal. This is because the amount ofinformation transmitted per symbol varies depending on the combinationof the modulation scheme and the channel coding rate. For a nextgeneration system, it is also being discussed to send and receivedifferent signals by multiple antennas provided at the sending andreceiving ends. In this case, control information such as schedulinginformation as described above may be necessary for each of the signalstransmitted by the multiple antennas. In other words, in such a system,the number of symbols necessary to transmit a control channel may varyfrom terminal to terminal and also vary depending on the number ofantennas used by the terminal. When the amount of information to betransmitted via a control channel varies from terminal to terminal, itis necessary to use a variable format that can flexibly accommodatevarious amounts of control information to improve resource useefficiency. However, using a variable format may increase the signalprocessing workload at the sending and receiving ends. Meanwhile, when afixed format is used, it is necessary to provide a dedicated controlchannel field that can accommodate the maximum amount of controlinformation. In this case, even if a control channel occupies only apart of the control channel field, the resources for the remaining partof the control channel field cannot be used for data transmission and asa result, the resource use efficiency is reduced. For these reasons,there is a demand for a method to transmit a control channel in a simpleand highly efficient manner.

However, related-art methods of transmitting a control channel stillcannot meet the above demands.

DISCLOSURE OF INVENTION Problems to be Solved by the Invention

One object of the present invention is to efficiently transmit controlchannels to communication terminals in a communication system where afrequency band allocated to the communication system includes multipleresource blocks each including one or more subcarriers and eachcommunication terminal communicates using one or more resource blocks.

Means for Solving the Problems

An aspect of the present invention provides a base station used in amobile communication system employing OFDM for downlink. The basestation includes a scheduler configured to determine allocation of radioresources for each subframe such that one or more resource blocks areallocated to each of selected user devices for communications; a controlchannel generating unit configured to generate a control channelincluding common control information to be mapped to radio resourcesdistributed across a system frequency band and specific controlinformation to be mapped to the one or more resource blocks allocated tothe each of the selected user devices; and a transmission signalgenerating unit configured to generate a transmission signal bytime-division-multiplexing the common control information and thespecific control information according to scheduling information fromthe scheduler. The common control information includes a formatindicator representing one of preset options that indicates the numberof symbols occupied by the common control information in one subframe.The common control information includes information units with apredetermined data size. The number of the information units is lessthan or equal to a specified multiplicity.

Advantageous Effect of the Invention

An aspect of the present invention makes it possible to efficientlytransmit control channels to communication terminals in a communicationsystem where a frequency band allocated to the communication systemincludes multiple resource blocks each including one or more subcarriersand each communication terminal communicates using one or more resourceblocks.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing used to describe frequency scheduling;

FIG. 2 is a drawing illustrating a frequency band used in an embodimentof the present invention;

FIG. 3A is a partial block diagram (1) of a base station according to anembodiment of the present invention;

FIG. 3B is a partial block diagram (2) of a base station according to anembodiment of the present invention;

FIG. 4A is a drawing illustrating signal processing components for onefrequency block;

FIG. 4B is a drawing illustrating signal processing components for onefrequency block;

FIG. 5A is a table showing exemplary information items of controlsignaling channels;

FIG. 5B is a drawing illustrating localized FDM and distributed FDM;

FIG. 5C is a drawing showing the number of symbols of an L1/L2 controlchannel which changes according to the number of multiplexed users;

FIG. 5D is a drawing illustrating exemplary mapping of part 0information and a paging indicator;

FIG. 5E is a drawing illustrating an information unit being used for apaging indicator;

FIG. 5F is a drawing illustrating a case where precoding vectors W_(A)and W_(B) are determined such that two of four streams are directed to auser device A (UE_(A)) and the other two of the four streams aredirected to a user device B (UE_(B));

FIG. 6 is a drawing illustrating the unit of error correction coding;

FIG. 7A is a drawing illustrating exemplary mapping of data channels andcontrol channels;

FIG. 7B is a drawing illustrating exemplary mapping of data channels andcontrol channels;

FIG. 7C is a drawing illustrating exemplary formats of an L1/L2 controlchannel in a case where the number of symbols of the L1/L2 controlchannel is reported using part 0;

FIG. 7D is a drawing illustrating an exemplary format of an L1/L2control channel in a case where the number of multiplexed users isreported for each MCS using part 0;

FIG. 7E is a drawing illustrating exemplary mapping of part 0 of anL1/L2 control channel in a three-sector configuration;

FIG. 7F is a drawing illustrating exemplary multiplexing schemes forgeneral control channels;

FIG. 7G is a drawing illustrating exemplary mapping of common controlinformation for users other than cell-edge users;

FIG. 7H is a drawing illustrating exemplary mapping of common controlinformation for users including cell-edge users;

FIG. 7I is a drawing illustrating an exemplary method of multiplexinggeneral control channels in a case where multiple users are multiplexed;

FIG. 8A is a partial block diagram of a terminal according to anembodiment of the present invention;

FIG. 8B is a partial block diagram of a terminal according to anembodiment of the present invention;

FIG. 8C is a block diagram illustrating a receiving unit of a terminal;

FIG. 9A is a flowchart showing an exemplary process according to anembodiment of the present invention;

FIG. 9B is a flowchart showing an exemplary parallel reception process;

FIG. 9C is a flowchart showing an exemplary serial reception process;

FIG. 10A is a drawing (1) illustrating error detection coding andchannel coding of general control channels;

FIG. 10B is a drawing (2) illustrating error detection coding andchannel coding of general control channels;

FIG. 10C is a drawing (3) illustrating error detection coding andchannel coding of general control channels;

FIG. 11 is a drawing illustrating an example of transmission powercontrol (TPC);

FIG. 12 is a drawing illustrating an example of adaptive modulation andcoding (AMC);

FIG. 13 is a drawing illustrating relationships between MCS levels anddata sizes;

FIG. 14A is a drawing illustrating transmission of L1/L2 controlchannels in four TTIs with various multiplicities;

FIG. 14B is a table showing exemplary values of parameters related tomultiplicity;

FIG. 15 is a drawing illustrating predetermined relative mappingpositions of control information;

FIG. 16 is a drawing used to describe a case where the number of blinddetection steps is reduced;

FIG. 17 is a table comparing methods 1 through 7;

FIG. 18 is a drawing (1) illustrating an example where a part of acontrol signal is coded using the same channel coding scheme for allusers and another part of the control signal is coded using differentchannel coding schemes for respective users;

FIG. 19A is a drawing (2) illustrating an example where a part of acontrol signal is coded using the same channel coding scheme for allusers and another part of the control signal is coded using differentchannel coding schemes for respective users;

FIG. 19B is a drawing used to describe methods of decoding a downlinkscheduling grant;

FIG. 20 is a drawing used to describe a case where the channel codingscheme for a control signal is varied from user to user;

FIG. 21 is a table comparing first through third methods;

FIG. 22 is a table showing exemplary data sizes of respectiveinformation items; and

FIG. 23 is a table comparing first through third methods.

EXPLANATION OF REFERENCES

-   -   31 Frequency block allocation control unit    -   32 Frequency scheduling unit    -   33-x Control signaling channel generating unit for frequency        block x    -   34-x Data channel generating unit for frequency block x    -   35 Broadcast channel (or paging channel) generating unit    -   1-x First multiplexing unit for frequency block x    -   37 Second multiplexing unit    -   38 Third multiplexing unit    -   39 Other channels generating unit    -   40 Inverse fast Fourier transform unit    -   41 Cyclic prefix adding unit    -   41 General control channel generating unit    -   42 Specific control channel generating unit    -   43 Multiplexing unit    -   81 Carrier frequency tuning unit    -   82 Filtering unit    -   83 Cyclic prefix removing unit    -   84 Fast Fourier transform unit (FFT)    -   85 CQI measuring unit    -   86 Broadcast channel decoding unit    -   87-0 General control channel (part 0) decoding unit    -   87 General control channel decoding unit    -   88 Specific control channel decoding unit    -   89 Data channel decoding unit

BEST MODE FOR CARRYING OUT THE INVENTION

According to an embodiment of the present invention, a control channelis divided into general control information (common control information)to be decoded by substantially all communication terminals and specificcontrol information to be decoded by specific communication terminalsthat are allocated one or more resource blocks, and the general controlinformation and the specific control information are encoded andmodulated separately. The control channel is generated bytime-division-multiplexing the general control information and thespecific control information according to scheduling information and istransmitted using a multicarrier scheme. This method makes it possibleto efficiently transmit a control channel using a fixed format withoutwasting resources even when the amount of control information variesfrom communication terminal to communication terminal

The general control information may be mapped so as to be distributedacross the entire system frequency band and the specific controlinformation for specific communication terminals may be mapped only toresource blocks allocated to the specific communication terminals. Inthis case, the specific control information is mapped to resource blocksthat provide good channel conditions for the respective specificcommunication terminals. Thus, this method makes it possible to improvethe quality of the specific control information while achieving acertain level of quality of the general control information for allusers.

A downlink pilot channel may also be mapped so as to be distributedacross multiple resource blocks allocated to multiple communicationterminals. Mapping a pilot channel across a wide band, for example,makes it possible to improve the accuracy of channel estimation.

According to an embodiment of the present invention, to maintain orimprove the reception quality of control channels including a generalcontrol channel and a specific control channel, transmission powercontrol is performed for the general control channel and one or both oftransmission power control and adaptive modulation and coding areperformed for the specific control channel.

Transmission power control may also be performed for the general controlchannel to improve the reception quality of the general control channelat specific communication terminals that are allocated resource blocks.Although all users or communication terminals receiving a generalcontrol channel try to demodulate the general control channel, it isenough if users who are allocated resource blocks can successfullydemodulate the general control channel.

The general control channel may include information on a modulationscheme and/or a coding scheme applied to the specific control channel.Since the combination of a modulation scheme and a coding scheme for thegeneral control channel is fixed (or is at least selected from a limitednumber of combinations), this method enables users who are allocatedresource blocks to obtain information on the modulation scheme and thecoding scheme for the specific control channel by demodulating thegeneral control channel. In other words, this method makes it possibleto perform adaptive modulation and coding on a specific control channelof a control channel and thereby to improve the reception quality of thespecific control channel.

When transmission power control and adaptive modulation and coding areperformed for control channels, the total number of combinations ofmodulation schemes and coding schemes for the specific control channelmay be less than the total number of combinations of modulation schemesand coding schemes for a shared data channel (physical downlink sharedchannel: PDSCH). This is because even if the required quality of thespecific control channel is not achieved solely by adaptive modulationand coding, there is no problem as long as the required quality can beachieved by additionally performing transmission power control.

First Embodiment

FIG. 2 is a drawing illustrating a frequency band used in an embodimentof the present invention. Values used in the descriptions below are justexamples and different values may be used. In this example, a frequencyband (entire transmission band) allocated to a communication system hasa bandwidth of 20 MHz. The entire transmission band includes fourfrequency blocks 1 through 4. Each of the frequency blocks includesmultiple resource blocks each including one or more subcarriers. FIG. 2schematically shows frequency blocks each including multiplesubcarriers. In this embodiment, it is assumed that four differentcommunication bandwidths of 5 MHz, 10 MHz, 15 MHz, and 20 MHz aredefined. A user device (e.g., a communication terminal, a mobileterminal, or a fixed terminal) performs communications using one or morefrequency blocks in one of the four bandwidths. A communication terminalin the communication system may support all of the four bandwidths orsupport only part of the four bandwidths. Still, each communicationterminal at least supports the 5 MHz bandwidth. Alternatively, nocommunication bandwidth may be defined and each communication terminalmay be configured to perform communications using the entire systemfrequency band. Although the above four communication bandwidths aredefined in this embodiment for descriptive purposes, the presentinvention may also be applied to a case where communication bandwidthsare not defined.

In this embodiment, a control channel (an L1/L2 control signalingchannel or a lower layer control channel) for reporting schedulinginformation of data channels (shared data channels) to terminals istransmitted using the minimum bandwidth (5 MHz) and is provided for eachfrequency block. For example, when a terminal supporting the 5 MHzbandwidth performs communications using frequency block 1, the terminalreceives a control channel provided for frequency block 1 and therebyobtains scheduling information. Information indicating which terminalscan use which frequency blocks may be reported in advance to theterminals, for example, via a broadcast channel. Also, the frequencyblocks used by the terminals may be changed after communications arestarted. When a terminal supporting the 10 MHz bandwidth performscommunications using adjacent frequency blocks 1 and 2, the terminalreceives control channels provided for frequency blocks 1 and 2 andthereby obtains scheduling information for the 10 MHz bandwidth. When aterminal supporting the 15 MHz bandwidth performs communications usingadjacent frequency blocks 1, 2, and 3, the terminal receives controlchannels provided for frequency blocks 1, 2, and 3 and thereby obtainsscheduling information for the 15 MHz bandwidth. When a terminalsupporting the 20 MHz bandwidth performs communications, the terminalreceives all control channels provided for the frequency blocks andthereby obtains scheduling information for the 20 MHz bandwidth. In FIG.2, four discrete blocks labeled “control channel” are shown in eachfrequency block. This indicates that a control channel is mapped(distributed) to multiple resource blocks in the frequency block.Details of control channel mapping are described later.

FIG. 3A is a partial block diagram of a base station according to anembodiment of the present invention. The base station shown in FIG. 3Aincludes a frequency block allocation control unit 31; a frequencyscheduling unit 32; a control signaling channel generating unit 33-1 anda data channel generating unit 34-1 for frequency block 1, . . . , and acontrol signaling channel generating unit 33-M and a data channelgenerating unit 34-M for frequency block M; a broadcast channel (orpaging channel) generating unit 35; a first multiplexing unit 1-1 forfrequency block 1, . . . , and a first multiplexing unit 1-M forfrequency block M; a second multiplexing unit 37; a third multiplexingunit 38; an other channels generating unit 39; an inverse fast Fouriertransform unit (IFFT) 40; and a cyclic prefix (CP) adding unit 41.

The frequency block allocation control unit 31 determines a frequencyblock(s) to be used by a terminal (a mobile terminal or a fixedterminal) based on information regarding the maximum supported bandwidthreported by the terminal. The frequency block allocation control unit 31manages the correspondence between respective terminals and frequencyblocks and sends the correspondence information to the frequencyscheduling unit 32. The correspondence between frequency blocks andterminals supporting different bandwidths may be reported in advance tothe terminals via a broadcast channel. For example, the frequency blockallocation control unit 31 allows a user communicating with the 5 MHzbandwidth to use any one or a specific one of frequency blocks 1 through4. For a user communicating with the 10 MHz bandwidth, the frequencyblock allocation control unit 31 allows the use of two adjacentfrequency blocks, i.e., frequency blocks “1 and 2”, “2 and 3”, or “3 and4”. The frequency block allocation control unit 31 may allow the user touse any one or a specific one of the combinations. For a usercommunicating with the 15 MHz bandwidth, the frequency block allocationcontrol unit 31 allows the use of three adjacent frequency blocks, i.e.,frequency blocks “1, 2, and 3” or “2, 3, and 4”. The frequency blockallocation control unit 31 may allow the user to use one or both of thecombinations. For a user communicating with the 20 MHz bandwidth, thefrequency block allocation control unit 31 allows the use of allfrequency blocks. Frequency blocks allowed to be used by a user may bechanged after communications are started according to a frequencyhopping pattern.

The frequency scheduling unit 32 performs frequency scheduling for eachof the frequency blocks. The frequency scheduling unit 32 performsfrequency scheduling for each frequency block based on channel qualityindicators (CQIs) reported by terminals for respective resource blockssuch that the resource blocks are allocated preferentially to terminalswith good channel conditions, and generates scheduling information basedon the scheduling results.

The control signaling channel generating unit 33-1 for frequency block 1forms a control signaling channel for reporting scheduling informationof frequency block 1 to terminals using only resource blocks withinfrequency block 1. Similarly, each of the control signaling channelgenerating units 33 for other frequency blocks forms a control signalingchannel for reporting scheduling information of the correspondingfrequency block to terminals using only resource blocks within thefrequency block.

The data channel generating unit 34-1 for frequency block 1 generatesdata channels each of which is to be transmitted using one or moreresource blocks in frequency block 1. Frequency block 1 may be shared byone or more terminals (users). Therefore, in this example, the datachannel generating unit 34-1 for frequency block 1 includes N datachannel generating units 1-1 through 1-N. Similarly, each of the datachannel generating units 34 for other frequency blocks generates datachannels for terminals sharing the corresponding frequency block.

The first multiplexing unit 1-1 for frequency block 1 multiplexessignals to be transmitted using frequency block 1. This multiplexingincludes at least frequency division multiplexing. Multiplexing of thecontrol signaling channel and the data channels is described later inmore detail. Similarly, each of the first multiplexing units 1 for otherfrequency blocks multiplexes a control signaling channel and datachannels to be transmitted using the corresponding frequency block.

The second multiplexing unit 37 changes positional relationships of thefirst multiplexing units 1-x (x=1, . . . , M) on the frequency axisaccording to a hopping pattern. Details of this process are described inthe second embodiment.

The broadcast channel (or paging channel) generating unit 35 generatesbroadcast information such as office data to be reported to terminalscovered by the base station. The broadcast information may includeinformation indicating the correspondence between maximum supportedbandwidths of terminals and usable frequency blocks. If the usablefrequency blocks are to be varied, the broadcast information may alsoinclude information specifying a hopping pattern indicating how theusable frequency blocks are to be varied. A paging channel may betransmitted using the same frequency band as that used for the broadcastchannel or using frequency blocks used by the respective terminals.

The other channels generating unit 39 generates channels other thancontrol signaling channels and data channels. For example, the otherchannels generating unit 39 generates a pilot channel.

The third multiplexing unit 38 multiplexes control signaling channelsand data channels of all frequency blocks, a broadcast channel, and/orother channels as necessary.

The inverse fast Fourier transform unit 40inverse-fast-Fourier-transforms a signal output from the thirdmultiplexing unit 38 and thereby modulates the signal according to OFDM.

The cyclic prefix (CP) adding unit 41 generates transmission symbols byattaching guard intervals to the OFDM-modulated symbols. A transmissionsymbol is, for example, generated by duplicating a series of data at theend (or head) of an OFDM-modulated symbol and attaching the duplicateddata to the head (or end) of the OFDM-modulated symbol.

FIG. 3B shows components following the CP adding unit 41 shown in FIG.3A. As shown in FIG. 3B, an RF transmission circuit performsdigital-analog conversion, frequency conversion, and band limitation onthe symbols with the guard intervals, and a power amplifier amplifiesthe symbols to an appropriate power level. Then, the symbols aretransmitted via a duplexer and a transceiver antenna.

In this embodiment, it is assumed that the base station performs antennadiversity reception using two antennas, although this feature is notessential for the present invention. An uplink signal received by thetwo antennas is input to an uplink signal receiving unit.

FIG. 4A is a drawing illustrating signal processing components for onefrequency block (xth frequency block). In FIG. 4A, “x” indicates aninteger greater than or equal to 1 and less than or equal to M. Signalprocessing components for frequency block x include a control signalingchannel generating unit 33-x, a data channel generating unit 34-x,multiplexing units 43-A, 43-B, . . . , and a multiplexing unit 1-x. Thecontrol signaling channel generating unit 33-x includes a generalcontrol channel generating unit 41 and one or more specific controlchannel generating units 42-A, 42-B, . . . .

The general control channel generating unit 41 performs channel codingand multilevel modulation on a general control channel (may also becalled general control information or common control information), whichis a part of a control signaling channel and is to be decoded anddemodulated by all terminals using the corresponding frequency block,and outputs the general control channel.

Each of the specific control channel generating units 42 performschannel coding and multilevel modulation on a specific control channel(may also be called specific control information), which is a part of acontrol signaling channel and is to be decoded and demodulated by aterminal to which one or more resource blocks in the correspondingfrequency block are allocated, and outputs the specific control channel.

The data channel generating unit 34-x includes data channel generatingunits x-A, x-B, . . . that, respectively, perform channel coding andmultilevel modulation on data channels of terminals A, B, . . . .Information regarding the channel coding and the multilevel modulationis included in the specific control channel described above.

The multiplexing units 43 map specific control channels and datachannels of respective terminals to resource blocks allocated to theterminals.

As described above, the general control channel generating unit 41encodes (and modulates) the general control channel and the specificcontrol channel generating units 42 encode (and modulate) the respectivespecific control channels. Accordingly, as schematically shown in FIG.6, the general control channel of this embodiment includes sets ofinformation for all users who are assigned frequency block x and thesets of information are collectively error-correction-coded.

Alternatively, the general control channel may be error-correction-codedfor each user. In this case, a user cannot uniquely identify a block inthe error-correction-coded blocks where the information for the user iscontained. Therefore, the user has to decode all blocks. With thismethod, because encoding is performed for each user, it is comparativelyeasy to add or change users. Each user has to decode and demodulate thegeneral control channel including the sets of information for all users.

Meanwhile, the specific control channels include only information forusers to which resource blocks are actually allocated and are thereforeerror-correction-coded for the respective users. Each user determineswhether a resource block(s) has been allocated by decoding anddemodulating the general control channel. Accordingly, only users whoare allocated resource blocks have to decode the specific controlchannels. The channel coding rates and modulation schemes for thespecific control channels are changed during communications as needed.On the other hand, the channel coding rate and the modulation scheme forthe general control channel may be fixed. Still, however, it ispreferable to perform transmission power control (TPC) to achieve acertain level of signal quality. Thus, error-correction-coded specificcontrol channels are transmitted using resource blocks providing goodchannel conditions. Therefore, the amount of downlink data may bereduced to some extent by puncturing. FIG. 5A shows types of downlinkcontrol signaling channels and exemplary information items of therespective downlink control signaling channels. Downlink controlsignaling channels include a broadcast channel (BCH), a dedicated L3signaling channel (upper layer control channel or high layer controlchannel), and an L1/L2 control channel (lower layer control channel).The L1/L2 control channel may include information for uplink datatransmission in addition to information for downlink data transmission.Also, the L1/L2 control channel may include a transmission format (e.g.,a data modulation scheme, a channel coding rate, and the number ofmultiplexed users) of the L1/L2 control channel. Information items to betransmitted by the respective channels are described below.

(Broadcast Channel)

The broadcast channel is used to report information that is unique to acell or information that changes only at long intervals to communicationterminals (either mobile terminals or fixed terminals; may also becalled user devices). For example, information that changes only at aninterval of 1000 ms (1 s) may be reported as broadcast information.Broadcast information may also include a transmission format of adownlink L1/L2 control channel, the maximum number of multiplexed users,resource block configuration information, and MIMO scheme information.The maximum number of multiplexed users indicates the number of userswhose control information is multiplexed in a downlink L1/L2 controlchannel in one subframe. The maximum number of multiplexed users may bespecified separately for uplink and downlink (N_(Umax) and N_(Dmax)) ormay be represented by the total number of multiplexed users for uplinkand downlink (N_(all)).

The transmission format is specified by a data modulation scheme and achannel coding rate. Since a channel coding rate can be uniquelydetermined based on a data modulation scheme and a data size, the datasize may be reported instead of the channel coding rate. Alternatively,the transmission format may be reported as a part (part 0) of an L1/L2control channel as described later.

The maximum number of multiplexed users indicates the number of usersthat can be multiplexed within one TTI using one or more of FDM, CDM,and TDM. The same maximum number of multiplexed users may be specifiedfor uplink and downlink, or different numbers may be specified foruplink and downlink.

The resource block configuration information indicates positions ofresource blocks used in a cell on the frequency and time axes. In thisembodiment, two types of frequency division multiplexing (FDM) schemesare used: localized FDM and distributed FDM. In localized FDM, aconsecutive frequency band locally-concentrated on the frequency axis isallocated preferentially to each user having good channel conditions.Localized FDM is suitable, for example, for communications of users withlow mobility and for high-quality, high-volume data transmission. Indistributed FDM, a downlink signal is generated such that it includesmultiple intermittent frequency components distributed across a widefrequency band. Distributed FDM is suitable, for example, forcommunications of users with high mobility and for periodic transmissionof small-size data such as voice packets (VoIP). Thus, frequencyresources are allocated as a consecutive frequency band or discretefrequency components to each user according to allocation informationbased on either of the FDM schemes.

The upper half of FIG. 5B illustrates an example of localized FDM. Inthis example, when a resource is identified by a localized resourceblock number “4”, it corresponds to physical resource block 4. The lowerhalf of FIG. 5B illustrates an example of distributed FDM. In thisexample, when a resource is identified by a distributed resource blocknumber “4”, it corresponds to left halves of physical resource blocks 2and 8. In the lower half of FIG. 5B, each physical resource block isdivided into two. However, the numbering and the number of divisions ofresource blocks in distributed FDM may vary from cell to cell. For thisreason, the resource block configuration information is reported via abroadcast channel to communication terminals in each cell.

The MIMO scheme information is reported if the base station is equippedwith multiple antennas and indicates whether single-user multi-inputmulti-output (SU-MIMO) or multi-user MIMO (MU-MIMO) is used. In SU-MIMO,a base station with multiple antennas communicates with onecommunication terminal with multiple antennas. Meanwhile, in MU-MIMO, abase station with multiple antennas communicates with pluralcommunication terminals.

In downlink MU-MIMO, a signal for a user device UE_(A) is transmittedfrom one or more antennas (e.g., a first antenna of two antennas) of thebase station and a signal for another user device UE_(B) is transmittedfrom other one or more antennas (e.g., a second antenna of the twoantennas) of the base station. In uplink MU-MIMO, a signal from a userdevice UE_(A) and a signal from another user device UE_(B) are receivedat the same time by multiple antennas of the base station. Signals frommultiple user devices are, for example, distinguished by referencesignals assigned to the respective user devices. As the referencesignals, CAZAC code sequences are preferably used. CAZAC code sequencesbecome orthogonal to each other even if they are generated from the samesequence as long as different cyclic shift amounts are applied.Therefore, orthogonal sequences can be easily generated by using CAZACcode sequences.

(Dedicated L3 Signaling Channel)

The dedicated L3 signaling channel is also used to report informationthat changes at long intervals, for example, at an interval of 1000 ms,to communication terminals. While the broadcast channel is sent to allcommunication terminals in a cell, the dedicated L3 signaling channel issent only to specific communication terminals. The dedicated L3signaling channel includes information on a type of FDM and persistentscheduling information. The dedicated L3 signaling channel may becategorized as a specific control channel.

The type of FDM indicates either localized FDM or distributed FDM isused for each of selected communication terminals.

The persistent scheduling information is reported when persistentscheduling is performed and indicates transmission formats (datamodulation schemes and channel coding rates) of uplink or downlink datachannels and resource blocks to be used.

(L1/L2 Control Channel)

The L1/L2 control channel may include information for uplink datatransmission in addition to information for downlink data transmission.The L1/L2 control channel may further include information bits (part 0)indicating the transmission format of the L1/L2 control channel.Information for downlink data transmission may be classified into part1, part 2 a, and part 2 b. Part 1 and part 2 a are categorized asgeneral control channels and part 2 b may be categorized as a specificcontrol channel.

(Part 0)

Part 0 information (hereafter simply called “part 0”) includes atransmission format (a modulation scheme, a channel coding rate, and thenumber of multiplexed users or a total number of control bits) of theL1/L2 control channel. If the transmission format of the L1/L2 controlchannel is reported by a broadcast channel, part 0 includes the numberof multiplexed users (or a total number of control bits).

The number of symbols necessary for the L1/L2 control channel variesdepending on the number of multiplexed users and the reception qualityof the users to be multiplexed. Typically, as shown in the left side ofFIG. 5C, a fairly large number of symbols are reserved for the L1/L2control channel. The number of symbols may be changed and reported bythe transmission format of the L1/L2 control channel, which is reportedvia the broadcast channel, for example, at an interval of about 1000 ms(1 s). When the number of multiplexed users is small, the number ofsymbols necessary for the L1/L2 control channel becomes smaller as shownin the right side of FIG. 5C. Here, if a large amount of resources iscontinuously reserved for the L1/L2 control channel in an environmentwhere the number of multiplexed users and the reception quality of themultiplexed users change at short intervals, a large part of theresources may be wasted.

To reduce the waste of resources for the L1/L2 control channel, part 0(a modulation scheme, a channel coding rate, and the number ofmultiplexed users or a total number of control bits) may be included inthe L1/L2 control channel. Reporting the modulation scheme and thechannel coding rate by part 0 of the L1/L2 control channel makes itpossible to change the modulation scheme and the channel coding rate atshorter intervals compared with a case where they are reported by thebroadcast channel. When the number of symbols occupied by the L1/L2control channel in one subframe is selected from preset options, thetransmission format can be identified by determining which one of theoptions is selected. For example, when four types of transmissionformats are provided as described later, the part 0 information may berepresented by two bits.

(Part 1)

Part 1 includes a paging indicator (PI). Each communication terminal candetermine whether it is being paged by demodulating the pagingindicator. More specifically, each communication terminal determineswhether a group number assigned to the communication terminal is presentin the paging indicator and demodulates a paging channel (PCH) if thegroup number is present. The positional relationship between the PI andthe PCH is known to the communication terminal. Then, the communicationterminal determines whether its identification information (e.g., thephone number of the communication terminal) is present in the PCH andthereby determines whether there is an incoming call.

The PI may be transmitted (1) using parts of the L1/L2 control channelthat are dedicated for the PI or (2) using non-dedicated informationunits in the L1/L2 control channel.

FIG. 5D illustrates a case where a paging indicator is transmittedaccording to the method (1). In the example shown in FIG. 5D, onesubframe includes a predetermined number (e.g., 10) oftemporally-consecutive OFDM symbols and the first three symbols areassigned to common control information. Part 0 information and a pagingindicator are mapped to frequency bands around the center frequency ofthe system frequency band according to distributed FDM. To other partsof the first three symbols, downlink (DL) control information and uplink(UL) control information are mapped according to distributed FDM. Apaging channel (PCH) is time-division-multiplexed with the above controlinformation. In this method, dedicated frequency bands are provided atregular or irregular intervals for the paging indicator.

In the method (2), the L1/L2 control channel includes multipleinformation units with a predetermined size. The number of informationunits is limited to the maximum number specified by broadcastinformation. Each of the information units normally contains controlinformation for a selected user device such as user identificationinformation (UE-ID) and resource allocation information. In this method,one or more of the information units are assigned to the pagingindicator at regular or irregular intervals. In other words, the pagingindicator is transmitted without using dedicated resources. In thiscase, however, it is necessary to appropriately distinguish aninformation unit containing the paging indicator from other informationunits containing control information for user devices. For this purpose,for example, identification information (PI-ID) unique to the pagingindicator may be used. In this case, the PI-ID is reported to userdevices, for example, by broadcast information.

The respective information units may have the same number of bits ordifferent numbers of bits. For example, when the MCS is variable anddetermined for each user in the common control information as describedlater (when the MCS for the L1/L2 control channel is adjusted for eachuser), the number of bits of an information unit may vary depending onthe MCS level.

FIG. 5E illustrates a case where information units are assigned to apaging indicator at regular or irregular intervals. When the user devicedecodes an information unit and detects a PI-ID, the user deviceprocesses the information unit as a paging indicator (the user devicedetermines whether a group ID assigned to itself is present in theinformation unit and checks the PCH if the group ID is present). Apaging indicator is preferably contained in the first information unitso that user devices can quickly determine whether incoming calls forthem are present.

(Part 2 a)

Part 2 a includes resource allocation information for downlink datachannels, an allocated time length, and MIMO information.

The resource allocation information for downlink data channelsidentifies resource blocks containing downlink data channels. For theidentification of resource blocks, various methods, such as a bitmapscheme and a tree numbering scheme, known in the relevant technicalfield may be used.

The allocated time length indicates a period of time for which downlinkdata channels are transmitted continuously. The resource allocation canbe changed as frequently as every TTI. However, to reduce the overhead,data channels may be transmitted according to the same resourceallocation for plural TTIs.

The MIMO information is reported when a MIMO scheme is used forcommunications and indicates, for example, the number of antennas andthe number of streams. The number of streams may also be called thenumber of information sequences. In the descriptions below, it isassumed that both of the number of antennas and the number of streamsare “four”. However, the number of antennas and the number of streamsmay take any appropriate value.

Although it is not essential, the whole or a part of 16-bit useridentification information may also be included in part 2 a.

(Part 2 b)

Part 2 b includes precoding information for a MIMO scheme, atransmission format of a downlink data channel, hybrid automatic repeatrequest (HARQ) information, and CRC information.

The precoding information for a MIMO scheme indicates weighting factorsapplied to respective antennas. Directional characteristics ofcommunication signals can be adjusted by adjusting the weighting factors(precoding vectors) to be applied to respective antennas. At thereceiving end (user terminal), channel estimation is preferablyperformed according to the directional characteristics.

FIG. 5F is a drawing illustrating a case where precoding vectors W_(A)and W_(B) are determined such that streams 1 and 2 (a code word 1) offour streams are directed to a user device A (UE_(A)) and streams 3 and4 (a code word 2) of the four streams are directed to a user device B(UE_(B)). A reference signal is transmitted in a non-directional manner.The precoding vectors W_(A) and W_(B) are reported to corresponding userdevices A and B. The user device A receives the reference signal takinginto account the weighting factor indicated by the precoding vectorW_(A) or applies the weighting factor to the reference signal after itis received. This configuration enables the user device A toappropriately perform channel estimation for a signal directed toitself. Similarly, the user device B receives the reference signaltaking into account the weighting factor indicated by the precodingvector W_(B) or applies the weighting factor to the reference signalafter it is received. This configuration enables the user device B toappropriately perform channel estimation for a signal directed toitself.

The transmission format of a downlink data channel is specified by adata modulation scheme and a channel coding rate. Since a channel codingrate can be uniquely determined based on a data modulation scheme and adata size, the data size or a payload size may be reported instead ofthe channel coding rate. For example, the transmission format may berepresented by 8 bits.

The hybrid automatic repeat request (HARQ) information includesinformation necessary for retransmission control of downlink packets.More specifically, the HARQ information includes a process number,redundancy version information indicating a packet combination scheme,and a new data indicator indicating whether a packet is a new packet ora retransmission packet. For example, the HARQ information may berepresented by 6 bits.

The CRC information is reported when a cyclic redundancy check isemployed for error detection and indicates CRC detection bits convolvedwith user identification information (UE-ID).

Information for uplink data transmission may be classified into part 1through part 4. Basically, information for uplink data transmission iscategorized as a general control channel. However, for communicationterminals that are allocated resources for downlink data channels, theinformation for uplink data transmission may be transmitted as specificcontrol channels.

(Part 1)

Part 1 includes delivery confirmation information for previous uplinkdata channels. The delivery confirmation information indicates eitheracknowledge (ACK) indicating that no error is detected in a packet or adetected error is within an acceptable range, or negative acknowledge(NACK) indicating an error out of the acceptable range is detected in apacket. The delivery confirmation information may be represented by onebit.

(Part 2)

Part 2 includes resource allocation information for a future uplink datachannel, and a transmission format, transmission power information, andCRC information for the uplink data channel.

The resource allocation information identifies resource blocks usablefor the transmission of the uplink data channel. For the identificationof resource blocks, various methods, such as a bitmap scheme and a treenumbering scheme, known in the relevant technical field may be used.

The transmission format of an uplink data channel is specified by a datamodulation scheme and a channel coding rate. Since a channel coding ratecan be uniquely determined based on a data modulation scheme and a datasize, the data size or a payload size may be reported instead of thechannel coding rate. For example, the transmission format may berepresented by 8 bits.

The transmission power information indicates a transmission power levelto be used for the transmission of an uplink data channel. According toan embodiment of the present invention, an uplink pilot channel isrepeatedly transmitted from each communication terminal to the basestation at a comparatively short interval Tref of, for example, aboutseveral milliseconds. A transmission power level Pref of the uplinkpilot channel is updated at an interval T_(TPC), which is longer thanthe interval Tref, based on transmission power control information (TPCcommand) from the base station such that the transmission power levelPref becomes greater or less than the transmission power level of apreviously-transmitted uplink pilot channel. An uplink L1/L2 controlchannel is transmitted with a transmission power level obtained byadding a first offset power level Δ_(L1L2) reported by the base stationto the transmission power level Pref of the uplink pilot channel. Anuplink data channel is transmitted with a transmission power levelobtained by adding a second offset power level Δ_(data) reported by thebase station to the transmission power level Pref of the uplink pilotchannel. The second offset power level Δ_(data) for a data channel isincluded in the transmission power information of part 2. The firstoffset power level Δ_(L1L2) for an L1/L2 control channel is included intransmission power information of part 4 described later. The TPCcommand for updating the transmission power level of the pilot channelis also included in part 4.

The first offset power level Δ_(L1L2) may be either a fixed value or avariable. When the first offset power level Δ_(L1L2) is a variable, itmay be reported to the user device as broadcast information (BCH) orlayer 3 signaling information. The second offset power level Δ_(data)may be reported to the user device via an L1/L2 control signal. Thefirst offset power level Δ_(L1L2) may be increased or decreasedaccording to the amount of information in a control signal. Also, thefirst offset power level Δ_(L1L2) may be determined according to thereception quality of a control signal. The second offset power levelΔ_(data) may be determined according to the reception quality of a datasignal. An uplink data channel may be transmitted with a transmissionpower level that is less than the sum of the transmission power levelPref of the uplink pilot channel and the second offset power levelΔ_(data) to comply with a request (overload indicator) to reduce powerconsumption which is sent from a cell around the serving cell of thecommunication terminal.

The CRC information is reported when a cyclic redundancy check isemployed for error detection and indicates CRC detection bits convolvedwith user identification information (UE-ID). In a response signal(downlink L1/L2 control channel) to a random access channel (RACH), arandom ID of the RACH preamble may be used as a UE-ID.

(Part 3)

Part 3 includes transmission timing control bits for uplink signals. Thetransmission timing control bits are used to synchronize communicationterminals in a cell. The transmission timing control bits may bereported as specific control information when resource blocks areallocated to a downlink data channel or may be reported as generalcontrol information.

(Part 4)

Part 4 includes transmission power information indicating a transmissionpower level of a communication terminal. Specifically, the transmissionpower information indicates a transmission power level to be used by acommunication terminal, which is not allocated resources for uplink datachannel transmission, to transmit an uplink control channel to report adownlink CQI. The offset power level Δ_(L1L2) and the TPC commanddescribed above are included in part 4.

FIG. 4B, like FIG. 4A, shows signal processing components for onefrequency block. FIG. 4B is different from FIG. 4A in that examples ofcontrol information are provided. In FIG. 4B, the same reference numbersare used for components corresponding to those in FIG. 4A. “Allocatedresource block mapping” in FIG. 4B indicates that channels are mapped toone or more resource blocks allocated to a selected communicationterminal. “Other resource block mapping” indicates that channels aremapped to resource blocks in the entire frequency block. Part 0 in theL1/L2 control channel is transmitted as a general control channel usingthe entire frequency block. Information regarding uplink datatransmission (part 1 through 4) in the L1/L2 control channel istransmitted as a specific control channel using resources allocated fora downlink data channel if available or transmitted as a general controlchannel using the entire frequency block if no resource is allocated fora downlink data channel.

FIG. 7A is a drawing illustrating exemplary mapping of data channels andcontrol channels. This example shows mapping within one frequency blockand one subframe and roughly corresponds to an output from the firstmultiplexing unit 1-x (except that channels such as a pilot channel aremultiplexed by the third multiplexing unit 38). One subframe maycorrespond to one transmission time interval (TTI) or to multiple TTIs.In this example, a frequency block includes seven resource blocks RB1through RB7. The seven resource blocks are allocated to terminals withgood channel conditions by the frequency scheduling unit 32 shown inFIG. 3A.

Normally, a general control channel, a pilot channel, and data channelsare time-division-multiplexed. The general control channel (includingpart 0 in the L1/L2 control channel) is mapped to resources distributedacross the entire frequency block. In other words, the general controlchannel is distributed across a frequency band composed of sevenresource blocks. In FIG. 7A, the general control channel (including part0 in the L1/L2 control channel) and other control channels (excludingspecific control channels) are frequency-division-multiplexed. The othercontrol channels may include a synchronization channel (such distinctionof channels is not essential for the present invention and asynchronization channel may be included in the general control channel).Part 0 in the L1/L2 control channel is preferably mapped to the firstOFDM symbol to reduce delay time. In the example shown in FIG. 7A, thegeneral control channel and the other control channels arefrequency-division-multiplexed such that each of the channels is mappedto multiple frequency components arranged at intervals. Such amultiplexing scheme is called distributed frequency divisionmultiplexing (FDM). Distributed FDM is preferable to achieve frequencydiversity gain. The frequency components allocated to the respectivechannels may be arranged at the same intervals or at differentintervals. In either case, it is necessary to distribute the generalcontrol channel across all resource blocks (in this embodiment, theentire frequency block). CDM may also be used as an additionalmultiplexing scheme to cope with the increase in the number ofmultiplexed users. CDM makes it possible to further increase thefrequency diversity gain. On the other hand, however, CDM may disruptthe orthogonality and reduce the reception quality.

In the example, the pilot channel is also mapped to frequency componentsdistributed across the entire frequency block. Mapping a pilot channelto a wide frequency range as shown in FIG. 7A is preferable toaccurately perform channel estimation for various frequency components.

In FIG. 7A, resource blocks RB1, RB2, and RB4 are allocated to user 1(UE1), resource blocks RB3, RB5, and RB6 are allocated to user 2 (UE2),and resource block RB7 is allocated to user 3 (UE3). As described above,resource block allocation information is included in the general controlchannel. A specific control channel for user 1 is mapped to thebeginning of resource block RB1 allocated to user 1. A specific controlchannel for user 2 is mapped to the beginning of resource block RB3allocated to user 2. A specific control channel for user 3 is mapped tothe beginning of resource block RB7 allocated to user 3. Note that, inFIG. 7A, the sizes of the portions occupied by the respective specificcontrol channels of users 1, 2, and 3 are not equal. This indicates thatthe amount of information of the specific control channel may varydepending on the user. The specific control channel is mapped locally toresources within a resource block allocated to a data channel. Incontrast with distributed FDM where a channel is mapped to resourcesdistributed across multiple resource blocks, this mapping scheme iscalled localized frequency division multiplexing (FDM).

FIG. 7B shows another exemplary mapping of specific control channels. InFIG. 7A, the specific control channel for user 1 (UE1) is mapped only toresource block RB1. In FIG. 7B, the specific control channel for user 1is mapped to resources discretely distributed across resource blocksRB1, RB2, and RB4 (across all the resource blocks allocated to user 1)by distributed FDM. The specific control channel for user 2 (UE2) isalso mapped to resources distributed across resource blocks RB3, RB5,and RB6 in a manner different from that shown in FIG. 7A. The specificcontrol channel and the shared data channel of user 2 aretime-division-multiplexed. Thus, a specific control channel and a shareddata channel of a user may be multiplexed in the whole or a part of oneor more resource blocks allocated to the user by time divisionmultiplexing and/or frequency division multiplexing (localized FDM ordistributed FDM). Mapping a specific control channel to resourcesdistributed across two or more resource blocks makes it possible toachieve frequency diversity gain also for the specific control channeland thereby to improve the reception quality of the specific controlchannel.

Exemplary formats of part 0 information in the L1/L2 control channel aredescribed below.

FIG. 7C shows exemplary formats of the L1/L2 control channel. In FIG.7C, four exemplary formats of the L1/L2 control channel are provided.The number of symbols (or the number of multiplexed users) of the L1/L2control channel differs from format to format. Information indicatingwhich one of the four formats is used is reported by the part 0information. When a modulation and coding scheme (MCS) reported by abroadcast channel to communication terminals are used for the L1/L2control channel, the number of symbols necessary for the L1/L2 controlchannel varies depending on the number of multiplexed users and the MCSlevel. To report the number of symbols, control bits (two bits in FIG.7C) are provided as part 0 information of the L1/L2 control channel. Forexample, when control bits 00 are reported as part 0 information, thecommunication terminal decodes the control bits and determines that thenumber of symbols of the L1/L2 control channel is 100. In FIG. 7C, thefirst two bits of each format corresponds to part 0 and a controlchannel with a variable length corresponds to the general controlchannel (part 1 and part 2 a for downlink). Instead of reporting the MCSvia a broadcast channel as in FIG. 7C, the MCS may be reported via an L3signaling channel.

FIG. 7D is a drawing illustrating an exemplary format of the L1/L2control channel in a case where the number of multiplexed users isreported for each MCS using part 0. In a case where an appropriate MCSis selected from predetermined MCSs according to the reception qualityof each communication terminal, the number of symbols necessary for theL1/L2 control channel varies depending on the reception quality of thecommunication terminal. To identify the reception quality, control bits(8 bits in FIG. 7D) are provided as part 0 information of the L1/L2control channel. In FIG. 7D, it is assumed that four types of MCSs areprovided and the maximum number of multiplexed users is three. Thenumber of multiplexed users 0 to 3 can be represented by two bits (00=0user, 01=1 user, 10=2 users, and 11=3 users). In this case, since twobits are necessary for each MCS, a total of 8 bits are necessary forpart 0. For example, when control bits 01100001 are reported as part 0information, each communication terminal determines control information(e.g., part 2 a for downlink) corresponding to its reception qualitybased on the control bits. In the example shown in FIG. 7D, 01100001indicates numbers of multiplexed users 1, 2, 0, and 1. In other words,assuming that the reception quality is expressed by four levels (lowest,low, middle, and high), 01100001 indicates reception quality levels oflow, middle, lowest, and high, and MCSs corresponding to the receptionquality levels are selected (a higher MCS level is selected and thenumber of multiplexed users increases as the reception quality levelincreases).

FIG. 7E shows exemplary mapping of information bits (part 0) of theL1/L2 control channel in a three-sector configuration. In a three-sectorconfiguration, three mapping patterns may be provided to transmitinformation bits (part 0) indicating transmission formats of the L1/L2control channel, and the mapping patterns may be assigned to therespective sectors such that those patterns do not overlap each other inthe frequency domain. Selecting different mapping patterns for adjacentsectors (or cells) makes it possible to achieve interferencecoordination.

FIG. 7F shows exemplary multiplexing schemes. In the above example,various general control channels are multiplexed by distributed FDM.However, any appropriate multiplexing scheme such as code divisionmultiplexing (CDM) or time division multiplexing (TDM) may be used. FIG.7F (1) shows an example of distributed FDM. In FIG. 7F (1), discretefrequency components identified by numbers 1, 2, 3, and 4 are used toproperly orthogonalize user signals. Discrete frequency components maybe arranged at regular intervals as exemplified or at irregularintervals. Also, different arrangement rules may be used for adjacentcells to randomize the interference when transmission power control isperformed. FIG. 7F (2) shows an example of code division multiplexing(CDM). In FIG. 7F (2), codes 1, 2, 3, and 4 are used to properlyorthogonalize user signals. CDM makes it possible to effectively reduceother cell interference. FIG. 7F (3) shows an example of distributed FDMwhere the number of multiplexed users is three. In FIG. 7F (3), discretefrequency components are redefined by numbers 1, 2, and 3 to properlyorthogonalize user signals. If the number of multiplexed users is lessthan the maximum number, the base station may be configured to increasethe transmission power of downlink control channels as shown in FIG. 7F(4). This method is preferable to increase the received signal quality,but may increase the other cell interference if transmission isperformed at a cell edge. A hybrid multiplexing scheme of CDM and FDMmay also be used.

Meanwhile, for transmission of the part 0 information, both of the MCS(a combination of a modulation scheme and a channel coding rate) and thetransmission power may be fixed, or only the MCS may be fixed while thetransmission power is varied. Also, the same part 0 information may beused for all users in a cell or the transmission format of the L1/L2control channel may be changed from user to user. For example, atransmission format for users located near the base station may beoptimized by appropriately changing the part 0 information and a fixedtransmission format may be used for users located near the cell edge. Inthis case, it is necessary to send information indicating whether usersbelong to a cell edge group to the users via, for example, a downlinkL1/L2 control channel. For a user not belonging to the cell edge group,a transmission format changed at intervals (e.g., every TTI) is reportedby the part 0 information; and for a user belonging to the cell edgegroup, a fixed transmission format is used to send the L1/L2 controlinformation.

FIG. 7G shows exemplary mapping of the L1/L2 control channel in a casewhere only users 1 through 4 located near the base station are in thecell. Numbers in FIG. 7G correspond to the respective users. Forexample, “1” corresponds to user 1. In this case, a transmission formatis reported, for example, for each TTI to users 1 through 4 by the part0 information. FIG. 7H shows exemplary mapping of the L1/L2 controlchannel in a case where users 1 through 4 located near the base stationand users 11 through 14 located at the cell edge are in the cell. Apredetermined transmission format is used for users 11 through 14, andthe transmission format is not explicitly reported to users 11 through14. Meanwhile, a transmission format that is the same as thepredetermined transmission format is reported to users 1 through 4 bythe part 0 information.

FIG. 7I illustrates an exemplary method of multiplexing general controlchannels in a case where multiple users are multiplexed. In this case,the L1/L2 control channel is mapped to resources within three OFDMsymbols in each subframe.

Subcarriers allocated to the L1/L2 control channel constitute multiplecontrol resource blocks. For example, one control resource block iscomposed of X subcarriers (X is an integer greater than 0). X is set atan optimum value according to, for example, the system bandwidth. Forthe control resource blocks, FDM or a hybrid of CDM and FDM is used asthe multiplexing scheme. When multiple OFDM symbols are used for theL1/L2 control channel, each control resource block is mapped to all ofthe OFDM symbols. The number of the control resource blocks is reportedvia a broadcast channel.

A control channel is data-modulated by QPSK or 16QAM. When multiplecoding rates (R1, R2, . . . , Rn) are used, Rn is represented by R1/n.

Even when uplink scheduling information and downlink schedulinginformation have different numbers of bits, control resource blocks withthe same size are used by employing rate matching.

FIG. 8A is a partial block diagram of a mobile terminal according to anembodiment of the present invention. The mobile terminal shown in FIG.8A includes a carrier frequency tuning unit 81, a filtering unit 82, acyclic prefix (CP) removing unit 83, a fast Fourier transform unit (FFT)84, a CQI measuring unit 85, a broadcast channel (or paging channel)decoding unit 86, a general control channel (part 0) decoding unit 87-0,a general control channel decoding unit 87, a specific control channeldecoding unit 88, and a data channel decoding unit 89.

The carrier frequency tuning unit 81 appropriately adjusts the centerfrequency of the reception band so as to be able to receive a signal ina frequency block allocated to the terminal.

The filtering unit 82 filters the received signal.

The cyclic prefix removing unit 83 removes guard intervals from thereceived signal and thereby extracts effective symbols from receivedsymbols.

The fast Fourier transform unit (FFT) 84 fast-Fourier-transformsinformation in the effective symbols and demodulates the informationaccording to OFDM.

The CQI measuring unit 85 measures the received power level of a pilotchannel in the received signal and feeds back the measurement as achannel quality indicator (CQI) to the base station. The CQI is measuredfor each resource block in the frequency block and all measured CQIs arereported to the base station.

The broadcast channel (or paging channel) decoding unit 86 decodes abroadcast channel. The broadcast channel decoding unit 86 also decodes apaging channel if it is included.

The general control channel (part 0) decoding unit 87-0 decodes part 0information in an L1/L2 control channel. Part 0 indicates thetransmission format of a general control channel.

The general control channel decoding unit 87 decodes a general controlchannel in the received signal and thereby extracts schedulinginformation. The scheduling information includes information indicatingwhether resource blocks are allocated to a shared data channel for theterminal and if resource blocks are allocated, also includes informationindicating the corresponding resource block numbers.

The specific control channel decoding unit 88 decodes a specific controlchannel in the received signal. The specific control channel includes adata modulation scheme, a channel coding rate, and HARQ information forthe shared data channel.

The data channel decoding unit 89 decodes the shared data channel in thereceived signal based on information extracted from the specific controlchannel. The terminal may be configured to report acknowledge (ACK) ornegative acknowledge (NACK) to the base station according to the resultof decoding.

FIG. 8B is also a partial block diagram of the mobile terminal. FIG. 8Bis different from FIG. 8A in that examples of control information areprovided. In FIG. 8B, the same reference numbers are used for componentscorresponding to those in FIG. 8A. “Allocated resource block demapping”in FIG. 8B indicates that information mapped to one or more resourceblocks allocated to the terminal is extracted. “Other resource blockdemapping” indicates that information mapped to resource blocks in theentire frequency block is extracted.

FIG. 8C shows components related to a receiving unit of the mobileterminal shown in FIG. 8A. In this embodiment, it is assumed that themobile terminal performs antenna diversity reception using two antennas,although this feature is not essential for the present invention.Downlink signals received by the two antennas are input to RF receptioncircuits 81 and 82. Cyclic prefix removing units 83 remove guardintervals (cyclic prefixes) from the signals, and fast Fourier transform(FFT) units 84 fast-Fourier-transform the signals. Then, the signals arecombined by an antenna diversity combining unit. The combined signal isinput to the respective decoding units shown in FIG. 8A or to aseparating unit shown in FIG. 8B.

FIG. 9A is a flowchart showing an exemplary process according to anembodiment of the present invention. In the descriptions below, it isassumed that a user carrying a mobile terminal UE1 supporting a 10 MHzbandwidth has entered a cell or a sector using a 20 MHz bandwidth forcommunications. It is also assumed that the minimum frequency band ofthe communication system is 5 MHz and the entire system frequency bandis divided into four frequency blocks 1 through 4 as shown in FIG. 2.

In step S11, the terminal UE1 receives a broadcast channel from the basestation and determines frequency blocks that the terminal UE1 is allowedto use. The broadcast channel is, for example, transmitted using a 5 MHzband including the center frequency of the 20 MHz band. This enablesterminals supporting different bandwidths to easily receive thebroadcast channel. For example, the base station allows a usercommunicating with a 10 MHz bandwidth to use a combination of twoadjacent frequency blocks, i.e., frequency blocks 1 and 2, 2 and 3, or 3and 4. The base station may allow the user to use any one or a specificone of the combinations. In this example, it is assumed that theterminal UE1 is allowed to use frequency blocks 2 and 3.

In step S12, the terminal UE1 receives a downlink pilot channel andmeasures the received signal quality for respective frequency blocks 2and 3. The received signal quality is measured for each resource blockin the respective frequency blocks and all measurements are reported aschannel quality indicators (CQIs) to the base station.

In step S21, the base station performs frequency scheduling for eachfrequency block based on CQIs reported by the terminal UE1 and otherterminals. In this example, a data channel for the terminal UE1 istransmitted using frequency blocks 2 and 3. This information is beingmanaged by the frequency block allocation control unit 31 (see FIG. 3).

In step S22, the base station generates a control signaling channel foreach frequency block according to scheduling information. The controlsignaling channel includes a common control channel (general controlchannel) and specific control channels.

In step S23, the base station transmits control channels and shared datachannels of the respective frequency blocks according to the schedulinginformation.

In step S13, the terminal UE1 receives signals transmitted via frequencyblocks 2 and 3.

In step S14, the terminal UE1 determines transmission formats of commoncontrol channels based on parts 0 of control channels received viafrequency blocks 2 and 3.

In step S15, the terminal UE1 separates the common control channel fromthe control channel received via frequency block 2, decodes the commoncontrol channel, and thereby extracts scheduling information. Similarly,the terminal UE1 separates the common control channel from the controlchannel received via frequency block 3, decodes the common controlchannel, and thereby extracts scheduling information. The schedulinginformation of each of frequency blocks 2 and 3 includes informationindicating whether resource blocks are allocated to a shared datachannel for the terminal UE1 and if resource blocks are allocated, alsoincludes information indicating the corresponding resource blocknumbers. If no resource block is allocated to any shared data channelfor the terminal UE1, the terminal UE1 returns to the standby mode andwaits for the next control channels. If resource blocks are allocated tothe shared data channel for the terminal UE1, the terminal UE1 separatesa specific control channel from the received signal and decodes thespecific control channel in step S16. The specific control channelincludes a data modulation scheme, a channel coding rate, and HARQinformation for the shared data channel.

In step S17, the terminal UE1 decodes the shared data channel in thereceived signal based on information extracted from the specific controlchannel. The terminal may be configured to report acknowledge (ACK) ornegative acknowledge (NACK) to the base station according to the resultof decoding. Thereafter, the above steps are repeated.

FIGS. 9B and 9C show details of steps S14 through S16 in FIG. 9A. FIG.9B is a flowchart showing an exemplary parallel reception process. Instep S1, the terminal UE1 checks part 0 information in the commoncontrol information. For example, the terminal UE1 checks the value oftwo bits representing the part 0 information and determines which one ofpredefined formats is selected for the L1/L2 control channel.

In step S2, the terminal UE1 determines, for example, the number ofsymbols of the L1/L2 control channel in one subframe based on thedetermined format. Here, it is assumed that maximum numbers ofmultiplexed users N_(Umax) and N_(Dmax) determined, respectively, foruplink and downlink have been reported to the terminal by broadcastinformation. The terminal UE1 calculates a data size per user based onthe number of symbols of the L1/L2 control channel in one subframe andthe maximum number of multiplexed users.

In each of steps S3-1 through S3-N_(Dmax), the terminal UE1 demodulatesan information unit having the data size per user calculated in step S2.Each information unit having the data size per user corresponds to theinformation unit mentioned in the descriptions of the paging indicator(with reference to FIG. 5E). In steps S3-1 through S3-N_(Dmax), theterminal UE1 demodulates information units regarding downlink controlinformation. In practice, the number of communicating users may be lessthan the maximum number of multiplexed users N_(Dmax). In this example,steps S3-1 through S3-N_(Dmax) are performed in parallel, and thereforethe time necessary to perform the steps equals the time necessary todemodulate one information unit.

In step S4, the terminal UE1 determines whether downlink controlinformation for itself is present.

In each of steps S5-1 through S5-N_(Umax), the terminal UE1 demodulatesan information unit having the data size per user calculated in step S2.In steps S5-1 through S5-N_(Umax), unlike steps S3-1 throughS3-N_(Umax), the terminal UE1 demodulates information units regardinguplink control information. The information units regarding downlinkcontrol information and the information units regarding uplink controlinformation may have the same data size or different data sizes. Also inthese steps, the number of communicating users may be less than themaximum number of multiplexed users N_(Umax). In this example, stepsS5-1 through S5-N_(Umax) are performed in parallel, and therefore thetime necessary to perform the steps equals the time necessary todemodulate one information unit.

In step S6, the terminal UE1 determines whether uplink controlinformation for itself is present.

In the above example, it is assumed that the maximum number ofmultiplexed users is specified separately for uplink and downlink.Meanwhile, there is a case where only a total number of multiplexedusers Nall for uplink and downlink is reported by broadcast information.In this case, the number for uplink and the number for downlinkconstituting the total number Nall are unknown. Therefore, steps S3 fordownlink must be performed for the total number Nall, and steps S5 foruplink must be performed for the total number Nall. Thus, in this case,the number of demodulation steps at the communication terminalincreases. On the other hand, however, the amount of broadcastinformation necessary to report the number of multiplexed users isreduced (the amount of information necessary to report Nall is less thanthe amount of information necessary to report N_(Dmax) and N_(Umax)).

FIG. 9C is a flowchart showing an exemplary serial reception process. Instep S1, as in FIG. 9B, the terminal UE1 checks part 0 information inthe common control information. In step S2, the terminal UE1 determines,for example, the number of symbols of the L1/L2 control channel in onesubframe based on a format determined in step S1. The terminal UE1calculates a data size per user based on the number of symbols of theL1/L2 control channel in one subframe and the maximum number ofmultiplexed users.

In step S3, the terminal UE1 initializes a parameter n indicating thenumber of calculations (n=0).

In step S4, the terminal UE1 demodulates an information unit having thedata size per user calculated in step S2. In this step, the terminal UE1demodulates an information unit regarding downlink control information.

In step S5, the terminal UE1 determines whether downlink controlinformation for itself has been obtained. If downlink controlinformation for the terminal UE1 has not been obtained, the terminal UE1proceeds to step S6 and increments the parameter n by 1. Then, theterminal UE1 repeats step S4 to demodulate another information unit. Theterminal UE1 repeats steps S4 through S6 until downlink controlinformation for itself is obtained or the parameter n reaches themaximum number N_(Dmax).

In step S7, the terminal UE1 reinitializes the parameter n indicatingthe number of calculations (n=0).

In step S8, the terminal UE1 demodulates an information unit having thedata size per user calculated in step S2. In this step, the terminal UE1demodulates an information unit regarding uplink control information.

In step S9, the terminal UE1 determines whether uplink controlinformation for itself has been obtained. If the uplink controlinformation for the terminal UE1 has not been obtained, the terminal UE1proceeds to step S10 and increments the parameter n by 1. Then, theterminal UE1 repeats step S8 to demodulate another information unit. Theterminal UE1 repeats steps S8 through S10 until uplink controlinformation for itself is obtained or the parameter n reaches themaximum number N_(Dmax), and then terminates the process.

In this example, demodulation of information units is performedserially. Therefore, the minimum time necessary for the demodulationsubstantially equals the time necessary to demodulate one downlinkinformation unit and one uplink information unit; and the maximum timenecessary for the demodulation substantially equals the time necessaryto demodulate N_(Dmax) downlink information units and N_(Dmax) uplinkinformation units.

Meanwhile, there is a case where only a total number of multiplexedusers N_(all) for uplink and downlink is reported by broadcastinformation. In this case, the number for uplink and the number fordownlink constituting the total number Nall are unknown. Therefore,steps S4 through S6 for downlink must be repeated for up to the totalnumber Nall, and steps S8 through S10 for uplink must be repeated for upto the total number Nall. Thus, in this case, the number of demodulationsteps at the communication terminal increases. On the other hand,however, the amount of broadcast information necessary to report thenumber of multiplexed users is reduced (the amount of informationnecessary to report Nall is less than the amount of informationnecessary to report N_(Dmax) and N_(Umax)).

Second Embodiment

Since the general control channel (including part 0) is informationnecessary for all users and is used to decode data channels, errordetection (CRC) coding and channel coding are performed on the generalcontrol channel. In a second embodiment of the present invention,exemplary methods of error detection coding and channel coding aredescribed. In the configuration of FIG. 4B, it is assumed that L1/L2control information (part 0) and L1/L2 control information (parts 2 aand 2 b) are channel-coded separately (i.e.,channel-coding/spreading/data-modulation units 41 and 42-A are provided,respectively, for part 0, part 2 a, and part 2 b). Variations of thisconfiguration are described below.

FIG. 10A illustrates a method where part 0 and parts 2 a and 2 b areerror-detection-coded together but are channel-coded separately. In thiscase, each of communication terminals UE1 and UE2 performs errordetection collectively on part 0 and parts 2 a and 2 b, and extracts anL1/L2 control channel for itself from parts 2 a and 2 b based on part 0.

Since the error detection (CRC) code for part 0 generally becomes largerelative to the control bits of part 0, this method makes it possible toreduce the overhead of error detection coding.

FIG. 10B illustrates a method where part 0 and parts 2 a and 2 b areerror-detection-coded and channel-coded separately. With this method,the overhead becomes greater compared with the case of FIG. 10A.However, this method eliminates the need to process parts 2 a and 2 bwhen error detection of part 0 fails.

FIG. 10C illustrates a method where part 0 and parts 2 a and 2 b areerror-detection-coded and channel-coded together. With this method, itis necessary to decode both part 0 and parts 2 a and 2 b to extract part0 information. However, this method improves the efficiency of channelcoding.

In the second embodiment, methods for error detection coding and channelcoding of part 0 and parts 2 a and 2 b are described with reference toFIGS. 10A through 10C. However, the above methods may also be applied toa general control channel other than parts 2 a and 2 b.

Third Embodiment

To improve the received signal quality of control channels, it ispreferable to perform link adaptation. In a third embodiment of thepresent invention, transmission power control (TPC) and adaptivemodulation and coding (AMC) are used to perform link adaptation. FIG. 11is a drawing illustrating an example of transmission power control wheretransmission power of downlink channels is controlled to achieve desiredreception quality. Referring to FIG. 11, a high transmission power levelis used to transmit a downlink channel to user 1 because user 1 is awayfrom the base station and its channel conditions are expected to bepoor. Meanwhile, channel conditions of user 2 close to the base stationare expected to be good. In this case, using a high transmission powerlevel to transmit a downlink channel to user 2 may increase the receivedsignal quality at user 2 but may also increase interference with otherusers. Because the channel conditions of user 2 are good, it is possibleto achieve desired reception quality with a low transmission powerlevel. Therefore, a downlink channel for user 2 is transmitted using acomparatively low transmission power level. When only transmission powercontrol is employed, a fixed combination of a modulation scheme and achannel coding scheme known to the sending and receiving ends are used.Accordingly, in this case, it is not necessary to report modulation andchannel coding schemes to be used to demodulate channels under thetransmission power control to the users.

FIG. 12 is a drawing illustrating an example of adaptive modulation andcoding (AMC) where one or both of the modulation scheme and the codingscheme are adaptively changed according to channel conditions to achievedesired reception quality. Assuming that the transmission power of thebase station is constant, it is expected that channel conditions of user1 away from the base station are poor. In such a case, the modulationlevel and/or the channel coding rate is set at a small value. In theexample shown in FIG. 12, QPSK is used as the modulation scheme for user1 and therefore two bits of information are transmitted per symbol. Onthe other hand, the channel conditions of user 2 close to the basestation are expected to be good and therefore, the modulation leveland/or the channel coding rate is set at a large value. In FIG. 12,16QAM is used as the modulation scheme for user 2 and therefore fourbits of information are transmitted per symbol. This method makes itpossible to achieve desired reception quality for a user with poorchannel conditions by improving the reliability, and to achieve desiredreception quality as well as increase the throughput for a user withgood channel conditions. When adaptive modulation and coding isemployed, modulation information including the modulation scheme, thecoding scheme, and the number of symbols of a received channel isnecessary to demodulate the channel. Therefore, it is necessary toreport the modulation information to the receiving end. Also, with theabove method, the number of bits transmitted per symbol varies dependingon the channel conditions. In other words, a small number of symbols arenecessary to transmit information when channel conditions are good, buta large number of symbols are necessary to transmit information whenchannel conditions are poor.

In the third embodiment of the present invention, transmission powercontrol is performed for a general control channel to be decoded by anyof users, and one or both of transmission power control and adaptivemodulation and coding are performed for specific control channels to bedecoded by selected users who are allocated resource blocks. The thirdembodiment may be implemented by any one of the three methods describedbelow.

(1) TPC-TPC

In a first method, only transmission power control is performed for thegeneral control channel and the specific control channels. In thismethod, a properly received channel can be demodulated without receivingmodulation information including the modulation scheme, coding rate,etc. in advance because they are fixed. The general control channel isdistributed across a frequency block and is therefore transmitted usingthe same transmission power throughout the entire frequency range.Meanwhile, a specific control channel for a user is mapped to resourceswithin a resource block allocated to the user. Therefore, transmissionpower of specific control channels may be adjusted for respective userswho are allocated resource blocks to improve the received signal qualityof the users. Taking FIGS. 7A and 7B as an example, the general controlchannel may be transmitted with a transmission power level P₀, thespecific control channel for user 1 (UE1) may be transmitted with atransmission power level P₁ suitable for user 1, the specific controlchannel for user 2 (UE2) may be transmitted with a transmission powerlevel P₂ suitable for user 2, and the specific control channel for user3 (UE3) may be transmitted with a transmission power level P₃ suitablefor user 3. In this case, shared data channels may be transmitted usingthe corresponding transmission power levels P₁, P₂, and P₃ or adifferent transmission power level P_(D).

As described above, the general control channel is decoded by all users.Also, the purpose of the general control channel is to report thepresence of data and the scheduling information of the data to users towhich resource blocks are allocated. Therefore, the transmission powerused to transmit the general control channel may be adjusted to achievedesired reception quality for the users who are allocated resourceblocks. For example, in FIGS. 7A and 7B, if all users 1, 2, and 3 whoare allocated resource blocks are located near the base station, thetransmission power level P₀ for the general control channel may be setat a comparatively small value. In this case, a user other than users 1,2, and 3 who is located, for example, at a cell edge may not be able todecode the general control channel properly. However, this does notcause any practical problem because no resource block is allocated tothe user.

(2) TPC-AMC

In a second method, transmission power control is performed for thegeneral control channel and adaptive modulation and coding is performedfor the specific control channels. When AMC is employed, it is basicallynecessary to provide users with modulation information in advance. Inthis method, modulation information for the specific control channels isincluded in the general control channel. Therefore, each user receives,decodes, and demodulates the general control channel first, anddetermines whether data for the user are present. If data for the userare present, the user extracts scheduling information as well asmodulation information including a modulation scheme, a coding scheme,and the number of symbols of the specific control channel. Then, theuser demodulates the specific control channel according to thescheduling information and the modulation information, thereby obtainsmodulation information of a shared data channel, and demodulates theshared data channel.

Control channels have a lower throughput requirement compared withshared data channels. Therefore, the number of combinations ofmodulation and coding schemes for AMC of the general control channel maybe smaller than that used for the shared data channel. For example, forAMC of the general control channel, QPSK is statically used as themodulation scheme and the coding rate may be selected from ⅞, ¾, ½, and¼.

The second method enables all users to receive the general controlchannel with a certain level of quality as well as to improve thereception quality of the specific control channels. This is achieved bymapping specific control channels to resource blocks providing goodchannel conditions for respective communication terminals and by usingappropriate modulation schemes and/or coding schemes for the respectivecommunication terminals. Thus, in this method, adaptive modulation andcoding is applied to specific control channels to improve theirreception quality.

When a very limited number of combinations of modulation schemes andchannel coding rates are used, a receiving end may be configured to tryall of the combinations to demodulate a specific control channel and touse properly demodulated information. This approach makes it possible toperform a certain level of AMC without reporting modulation informationto users in advance.

(3) TPC-TPC/AMC

In a third method, transmission power control is performed for thegeneral control channel, and both transmission power control andadaptive modulation and coding are performed for the specific controlchannels. As described above, when AMC is employed, it is basicallynecessary to provide users with modulation information in advance. Also,it is preferable to provide a large number of combinations of modulationschemes and channel coding rates to achieve desired reception qualityeven when the degree of fading is high. However, using a large number ofcombinations complicates the process of determining an appropriatecombination, increases the amount of information needed to report thedetermined combination, and thereby increases the processing workloadand overhead. In the third method, reception quality is maintained by acombination of TPC and AMC. In other words, it is not necessary tocompensate for the entire fading solely by AMC. For example, amodulation scheme and a coding scheme that nearly achieve desiredquality are selected and then transmission power is adjusted to fullyachieve the desired quality under the selected modulation scheme andcoding scheme. This method makes it possible to reduce the number ofcombinations of modulation schemes and channel coding schemes.

In all of the three methods described above, only transmission powercontrol is performed for the general control channel. Therefore, theuser can receive the general control channel with desired receptionquality and also can easily obtain control information from the generalcontrol channel. Unlike AMC, transmission power control does not changethe amount of information transmitted per symbol and therefore thegeneral control channel can be easily transmitted using a fixed format.Also, because the general control channel is distributed across theentire frequency block or multiple resource blocks, high frequencydiversity gain can be expected. This in turn makes it possible toachieve enough reception quality by simple transmission power controlwhere a long-period average of the transmission power level is adjusted.However, performing only transmission power control for the generalcontrol channel is not an essential feature of the present invention.For example, the transmission format of the general control channel maybe changed at long intervals and reported via a broadcast channel.

Meanwhile, including AMC control information (modulation information)for specific control channels in the general control channel makes itpossible to perform AMC for the specific control channels and therebymakes it possible to improve the transmission efficiency and quality ofthe specific control channels. While the number of symbols necessary fora general control channel is substantially constant, the number ofsymbols necessary for a specific control channel varies depending on themodulation scheme, the coding rate, the number of antennas, and so on.For example, assuming that the number of necessary symbols is N when thechannel coding rate is ½ and the number of antennas is 1, the number ofnecessary symbols becomes 4N when the channel coding rate is ¼ and thenumber of antennas is 2. With this embodiment, it is possible totransmit a control channel using a simple fixed format as shown in FIGS.7A and 7B even if the number of symbols necessary for the controlchannel changes. Although the number of symbols necessary for a specificcontrol channel changes, the number of symbols necessary for a generalcontrol channel does not change. Therefore, it is possible to flexiblycope with the variation in the number of symbols by changing theproportion of the specific control channel to the shared data channel ina given resource block.

Fourth Embodiment

Transmission formats of data channels are reported via the L1/L2 controlchannel. Therefore, the transmission format of the L1/L2 control channelmust be known to user devices. The simplest method to achieve this is touse one fixed transmission format for the L1/L2 control channel for allusers in a cell. However, for efficient use of radio resources and forlink adaptation, it is preferable to adaptively change even thetransmission format of the L1/L2 control channel from user to user. Inthis case, it is necessary to report a selected transmission format toeach user device. In a fourth embodiment of the present invention, thetransmission format of the L1/L2 control channel is adaptively changed.

Generally, the data size necessary to transmit information variesdepending on the transmission format used even if the number ofinformation bits to be transmitted is constant. A transmission format isspecified by parameters including a combination of a modulation schemeand a channel coding scheme (MCS information). The MCS information mayalso be specified by a combination of a modulation scheme and a datasize.

Referring to FIG. 13, the data size necessary to transmit informationusing MCS-2 (modulation scheme=QPSK, channel coding scheme R=¼) is twiceas large as the data size necessary to transmit the same informationusing MCS-1 (modulation scheme=QPSK, channel coding scheme R=½). Also,the data size necessary to transmit information using MCS-3 (modulationscheme=QPSK, channel coding scheme R=⅙) is three times as large as thedata size necessary to transmit the same information with MCS-1(modulation scheme=QPSK, channel coding scheme R=½). Thus, when the MCSto be applied to the L1/L2 control channel changes, the data size of theL1/L2 control channel changes. If the MCS is unknown in a decodingprocess, it may be necessary to repeat the decoding process for up tothe number of possible MCSs. Also, in the decoding process performed foreach possible MCS, a user device needs information indicating the numberof multiplexed users whose control information is multiplexed in theL1/L2 control channel to determine whether control information for theuser device is present (the user device can extract the controlinformation for itself, if available, by decoding information units forup to the number of multiplexed users).

As described in the first embodiment with reference to FIGS. 9B and 9C,the number of multiplexed users in the L1/L2 control channel may bereported to user devices separately for uplink and downlink or may bereported as the total number of multiplexed users for uplink anddownlink. The amount of radio resources necessary to report the numberof multiplexed users and the processing workload at the user devicesvary depending on which one of the two methods is used.

Before describing various methods according to the fourth embodiment,definitions of symbols (parameters) to be used in the descriptions aregiven.

-   -   N_(MCS) indicates the number of MCSs provided for the L1/L2        control channel. Combinations of data modulation schemes and        channel coding schemes used for the L1/L2 control channel are        represented by MCS-1 through MCS-N_(MCS).    -   N_(L1L2(max)) (=N′_(U)+N′_(D)) indicates the maximum number of        L1/L2 control channels that can be multiplexed in one TTI (when        the most efficient MCS is used).    -   N_(UE,D)(m) indicates the number of users using MCS-m in        downlink (a smaller number (m) is assigned to an MCS with higher        transmission efficiency).    -   N_(UE,D)(m) indicates the number of users using MCS-m in uplink        (a smaller number (m) is assigned to an MCS with higher        transmission efficiency).    -   N_(D) indicates the number of multiplexed L1/L2 control channels        related to downlink transmission (N′_(D) indicates the value of        N_(D) when an MCS with the highest transmission efficiency is        used).    -   N_(U) indicates the number of multiplexed L1/L2 control channels        related to uplink transmission (N′_(U) indicates the value of        N_(U) when an MCS with the highest transmission efficiency is        used).    -   N_(Dmax) indicates the maximum number of multiplexed L1/L2        control channels related to downlink transmission        (N_(D)□N_(Dmax))    -   N_(Umax) indicates the maximum number of multiplexed L1/L2        control channels related to uplink transmission        (N_(U)□N_(Umax)).

N_(L1L2(max)) indicates the maximum number of multiplexed L1/L2 controlchannels in any subframe and (N′_(D)+N′_(D)□ indicates the maximumnumber of multiplexed L1/L2 control channels in a specific subframe.

FIG. 14A is a drawing illustrating transmission of downlink L1/L2control channels in four TTIs with various multiplicities. FIG. 14Bshows exemplary values of the above defined parameters in associationwith FIG. 14A. In FIG. 14A, “D” indicates information related todownlink and “U” indicates information related to uplink. As shown inFIG. 14A, the data size of information varies according to the MCSapplied. In FIGS. 14A and 14B, for brevity, only two types of MCSs areprovided (the transmission efficiency of MCS-1 is higher than that ofMCS-2). Supposing that MCS-1 is used for all users, information for nineusers can be transmitted in a frequency band used in TTI-1. Regardingdownlink, D3 uses MCS-1 with high transmission efficiency and D1 and D2use MCS-2 with low transmission efficiency (in FIG. 14B, N_(UE,D)-MCS-1is 1 and N_(UE,D)-MCS-2 is 2). As described above with reference to FIG.13, the data size decreases as the efficiency of MCS increases.Regarding uplink, U2 and U3 use MCS-1 with high transmission efficiencyand U1 uses MCS-2 with low transmission efficiency (in FIG. 14B,N_(UE,D)-MCS-1 is 2 and N_(UE,D)-MCS-2 is 1). In TTI-1, although up tofive users can be multiplexed for downlink (N′_(D)=5), only three usersare actually multiplexed (N_(D)=3). Also in TTI-1, although up to fourusers can be multiplexed for uplink (N′_(U)=4), only three users areactually multiplexed (N_(U)=3). Exemplary values of parameters for otherTTIs are also shown in FIG. 14B.

Below, methods 1 through 7 of reporting the number of multiplexed usersto user devices are described. In the descriptions below, it is assumedthat the transmission format (i.e., the MCS number) of the L1/L2 controlchannel is changed from user to user. Characteristics of the respectivemethods are shown in FIG. 17.

(Method 1)

In method 1, the numbers of multiplexed users for each MCS (N_(UE,U)(m)and N_(UE,D)(m)) are reported for each TTI to user devices. With thismethod, the number of multiplexed users is reported separately foruplink and downlink. Therefore, a user device can extract controlinformation for itself (if available) by performing a decoding processfor up to N_(UE,U)(m)+N_(UE,D)(m) times (the number of times may becalled the number of blind detection steps). This method also makes itpossible to freely set the value of MCS-m for each user and thereforecan most efficiently transmit the L1/L2 control channel (enables themost efficient use of radio resources). Since the number of symbolsnecessary for the L1/L2 control channel is reported by part 0information, the boundary between the L1/L2 control channel and a shareddata channel can be changed for each TTI.

(Method 2)

Also in method 2, the MCS of the L1/L2 control channel is adjusted everyTTI for each user. In this method, the numbers of multiplexed L1/L2control channels for uplink and downlink (N′_(U) and N′_(D): valuesbased on the most efficient MCS) are determined separately and arereported for each TTI to user devices. Although the MCS is adjustedevery TTI for each user, MCS numbers selected for respective userdevices are not reported. Therefore, the number of blind detection stepsis represented by N_(MCS)×(N′_(U)+N′_(D)).

With this method, although the number of blind detection steps becomesfar greater than that in method 1, the number of bits necessary torepresent the numbers of multiplexed L1/L2 control channels can bereduced. Thus, this method is preferable in terms of reducing the numberof bits of part 0 information. Also, since the MCS is adjusted every TTIfor each user, method 2 makes it possible to use radio resources asefficiently as in method 1.

(Method 3)

Also in method 3, the MCS of the L1/L2 control channel is adjusted everyTTI. In this method, the total number of multiplexed L1/L2 controlchannels for uplink and downlink (N′_(U)+N′_(D): a total number based onthe most efficient MCS) is reported for each TTI to user devices.Although the MCS is adjusted every TTI for each user, MCS numbersselected for respective user devices are not reported. Therefore, thenumber of blind detection steps is represented by2×N_(MCS)×(N′_(U)+N′_(D)).

With this method, although the number of blind detection steps becomeseven greater than that in method 2 (two times larger than that in method2), the number of bits of part 0 information can be further reduced.Also, since the MCS is adjusted every TTI for each user, method 3 makesit possible to use radio resources as efficiently as in method 1.

(Method 4)

In method 4, the MCS of each user is not adjusted every TTI, but isadjusted at longer intervals and reported via an upper layer (e.g., byL3 control information). Meanwhile, the number of multiplexed users isreported for each TTI separately for uplink and downlink. The MCS ofeach user is adjusted at longer intervals than in methods 1 through 3.Therefore, transmission power control is preferably employed to preventdecrease in reception quality due to instantaneous fading. In thismethod, the numbers of multiplexed L1/L2 control channels for uplink anddownlink (N′_(U) and N′_(D): values based on the most efficient MCS) aredetermined separately and are reported for each TTI to user devices. Thenumber of blind detection steps, although it depends on the MCS, becomesless than or equal to N′_(U)+N′_(D).

With this method, since the MCS of each user is reported only at longintervals, it is possible to make the number of bits of part 0information smaller than that in method 1. Meanwhile, since the MCS isnot updated frequently, the use efficiency of radio resources may becomelower than that in method 1.

(Method 5)

Also in method 5, the MCS of each user is not adjusted every TTI, but isadjusted at longer intervals and reported via an upper layer (e.g., byL3 control information). Meanwhile, the total number of multiplexedusers for uplink and downlink is reported for each TTI. As in method 4,since the MCS of each user is adjusted only at long intervals,transmission power control is preferably employed to prevent decrease inreception quality due to instantaneous fading. In this method, the totalnumber of multiplexed L1/L2 control channels for uplink and downlink(N′_(U)+N′_(D): a total number based on the most efficient MCS) isreported for each TTI to user devices. Therefore, the number of blinddetection steps, although it depends on the MCS, becomes less than orequal to 2×(N′_(U)+N′_(D)).

Since the MCS is not updated frequently in this method, the useefficiency of radio resources is substantially the same as that inmethod 4. With method 5, since the number of multiplexed users isreported collectively for uplink and downlink, the number of blinddetection steps increases, but the number of bits of part 0 informationbecomes smaller than that in method 4.

(Method 6)

Also in method 6, the MCS of each user is not adjusted every TTI, but isadjusted at longer intervals and reported via an upper layer (e.g., byL3 control information). In this method, the total maximum number ofmultiplexed L1/L2 control channels for uplink and downlink is reportedfor each TTI to user devices and the maximum numbers of multiplexedL1/L2 control channels determined separately for uplink and downlink(N_(Umax) and N_(Dmax)) are reported, via an upper layer (e.g., via abroadcast channel (BCH)), to user devices at an interval longer than theTTI. Since the MCS of each user is adjusted only at long intervals,transmission power control is preferably employed to prevent decrease inreception quality due to instantaneous fading. The number of multiplexedL1/L2 control channels to be reported for each TTI is represented by atotal maximum number (N′_(U)+N′_(D)) obtained based on the mostefficient MCS.

In this method, relative mapping positions (arrangement of radioresources) of uplink control information and downlink controlinformation are predetermined. For example, downlink control channelsfor respective users are first mapped in sequence and then uplinkcontrol channels for respective users are mapped in sequence. In theexample shown in FIG. 15, a mapping scheme indicated by “∘” is allowedbut a mapping scheme indicated by “x” is prevented. Although anyappropriate mapping scheme other than that shown in FIG. 15 may be used,it is necessary to determine and fix the mapping scheme in advance.Fixing the relative mapping positions in advance makes it possible toreduce the number of blind detection steps.

In FIG. 16, areas surrounded by dotted lines indicate information unitsto be decoded in blind detection in a case where N_(Dmax)=6, N_(Umax)=4,and N_(D)+N_(U)=9. The user device does not have to perform blinddetection for areas not surrounded by dotted lines. Thus, determiningthe relative mapping positions of uplink and downlink controlinformation in advance makes it possible to reduce the number of blinddetection steps to be performed by the user device.

Since the MCS is not updated frequently in this method, the useefficiency of radio resources is substantially the same as that inmethod 4. With method 6, since the number of multiplexed users isreported collectively for uplink and downlink, the number of bits ofpart 0 information becomes smaller than that in method 4.

□Method □□

In method 7, a fixed MCS is used for all users in a cell. In thismethod, the total maximum number of multiplexed L1/L2 control channelsfor uplink and downlink is reported for each TTI to user devices and themaximum numbers of multiplexed L1/L2 control channels determinedseparately for uplink and downlink (N_(Umax) and N_(Dmax)) are reported,via an upper layer (e.g., via a broadcast channel (BCH)), to userdevices at an interval longer than the TTI.

As in method 6, it is possible to reduce the number of blind detectionsteps to be performed by the user device by determining the relativemapping positions of uplink and downlink control information in advance.With method 7, since the same fixed MCS is used for all users, the useefficiency of radio resources may become lower than other methods.However, since the number of multiplexed users is reported collectivelyfor uplink and downlink, the number of bits of part 0 informationbecomes smaller than that in method 4.

Fifth Embodiment

As described above, when a MIMO scheme is employed, the number ofcontrol bits necessary for downlink data transmission information(downlink scheduling grant information) including precoding vectors,transmission formats, and HARQ information may vary depending on theMIMO scheme selected. This is because the number of streams, the numberof code words, and the number of frequency-selective precoding vectorschange depending on the MIMO scheme.

Here, for such downlink scheduling grant information requiring avariable number of control bits, it is preferable to select a channelcoding scheme that enables efficient transmission (which leads to highercoding gain), fast decoding (in the fastest case, with only one decodingprocess), and reduction of the number of blind detection steps (by usinga fixed or known coding block size). Channel coding methods are outlinedabove with reference to FIG. 6. In a fifth embodiment of the presentinvention, channel coding methods are described in more detail.

Below, three channel coding methods for the downlink scheduling grantinformation are described.

(First Method)

FIG. 18 is a drawing illustrating an example where a part of a controlsignal is coded using the same channel coding scheme for all users andanother part of the control signal is coded using different channelcoding schemes for respective users. In a first method, a control signalis divided into a basic part with a basic data size and an additionalpart. The basic data size is determined such that the basic part cancontain all information necessary for the transmission of one stream.The same channel coding scheme is applied to users requiring onlycontrol information within the basic part. If the number of streams isgreater than 1, an additional part is provided in addition to the basicpart. The data size of the additional part may vary from user to user.Therefore, the additional part is coded using different coding schemesfor respective users (of course, there is a case where the same channelcoding scheme is applied to some of the users by chance). When receivinga control signal, the user device first decodes the basic part andthereby obtains control information. Then, if it is determined thatcontrol information for more than one stream is present for the userdevice, the user device decodes the additional part and thereby obtainsall the control information for multiple streams. With this method, auser device with only one stream has to repeat the decoding process onlyonce. Also, this method makes it possible to improve the codingefficiency even when the amount of control information varies from userto user.

(Second Method)

FIG. 19A is a drawing illustrating another example where a part of acontrol signal is coded using the same channel coding scheme for allusers and another part of the control signal is coded using differentchannel coding schemes for respective users. In a second method, thebasic data size is fixed and is smaller than that in the first method.In the first method, the amount of control information necessary for thetransmission of one stream may vary. In the second method, controlinformation is divided into a fixed-length part and a variable-lengthpart that are predetermined in the system. The fixed-length part mayinclude downlink resource allocation information and the number ofstreams. The variable-length part may include precoding information,transmission formats, and HARQ information for all streams. Like thefirst method, the second method also makes it possible to improve codingefficiency.

FIG. 19B is a drawing used to describe methods of decoding a downlinkscheduling grant at the user device in a case where a part of a controlsignal is coded using the same channel coding scheme for all users andanother part of the control signal is coded using different channelcoding schemes for respective users.

Option 1: the Basic Part and the Additional Part are Decoded Separately.

In this case, the additional part is mapped to a control resource blockthe index of which is predetermined. In the example shown in FIG. 19B,the basic part is mapped to a first block and the additional part ismapped to a second block located next to the first block. As the secondblock, a resource block allocated to a shared data channel may be used.

Option 2: the Fixed-Length Part and the Variable-Length Part are DecodedSeparately.

In the example shown in FIG. 19B, the basic part is mapped to the firstblock and the additional part is mapped to a predetermined resourceblock such as a control resource block or to a part of a resource blockallocated to a shared data channel.

(Third Method)

FIG. 20 is a drawing used to describe a case where the channel codingscheme for a control signal is varied from user to user. In the thirdmethod, the channel coding scheme is basically determined for each user(although there is a possibility that the same channel coding scheme isused for all users because of similar communication conditions). Allcontrol information items including a variable amount of controlinformation regarding MIMO are collectively channel-coded user by user.This method makes it possible to lengthen the unit of channel coding foreach user device and thereby makes it possible to achieve high codinggain.

FIG. 21 is a table comparing the first through third methods.

FIG. 22 is a table showing exemplary data sizes of respectiveinformation items.

FIG. 23 is a table comparing the first through third methods in terms ofthe number of symbols. More specifically, FIG. 23 shows the number ofsymbols necessary for the downlink scheduling grant information for eachmethod in a case where data sizes of precoding information, transmissionformat information, and HARQ information are fixed to reduce the numberof blind detection steps. In the example shown in FIG. 23, data sizesshown in FIG. 22 are used to calculate the number of symbols. In thefirst method, CRC information is attached only to the basic part (inother words, the CRC information is calculated based on both of thebasic part and the additional part). QPSK and R=½ are used as themodulation scheme and the channel coding scheme (MCS) for the downlinkscheduling grant information. The number of bits (B) of the precodingvector information and the number of code words N_(codeword) are variedas parameters.

As shown in FIG. 23, when the number of control bits of the precodinginformation is small (case A), the overhead in the first method isslightly larger than that in the second method but the difference isignorable. Meanwhile, the overhead in the third method increases up toabout 30% when the frequency band is 5 MHz and increases up to about 16%when the frequency band is 20 MHz. When the number of control bits ofthe precoding information is large (case B), the overhead in the firstand third methods becomes greater than that in the second method.

The present invention is not limited to the specifically disclosedembodiments, and variations and modifications may be made withoutdeparting from the scope of the present invention. Although specificvalues are used in the above descriptions to facilitate theunderstanding of the present invention, the values are just examples anddifferent values may also be used unless otherwise mentioned. Thedistinctions between the embodiments are not essential for the presentinvention, and the embodiments may be used individually or incombination. Although functional block diagrams are used to describeapparatuses in the above embodiments, the apparatuses may be implementedby hardware, software, or a combination of them.

The present international application claims priority from JapanesePatent Application No. 2007-001862 filed on Jan. 9, 2007 and JapanesePatent Application No. 2007-073732 filed on Mar. 20, 2007, the entirecontents of which are hereby incorporated herein by reference.

The invention claimed is:
 1. A base station for a mobile communicationsystem employing orthogonal frequency division multiplexing (OFDM) fordownlink and using subframes each including multiple OFDM symbols, thebase station comprising: a mapping unit configured to map a controlchannel to a predetermined number of OFDM symbols from a beginning ofeach subframe and to map a data channel to OFDM symbols that follow theOFDM symbols to which the control channel is mapped; and a transmittingunit configured to transmit the control channel and the data channelmapped by the mapping unit, wherein multiple control resource blocks aremultiplexed in the control channel mapped by the mapping unit and eachof the control resource blocks is mapped to every one of the OFDMsymbols to which the control channel is mapped; and a fixed transmissionformat is used for transmission of the control channel to some userdevices in a cell and various transmission formats are used fortransmission of the control channel to other user devices in the cell.2. The base station as claimed in claim 1, wherein the control channelincludes paging indicator identification information that is differentfrom user device identification information.
 3. The base station asclaimed in claim 1, wherein for each subframe, combinations ofmodulation schemes and channel coding schemes to be applied toinformation units provided for the respective user devices in thecontrol channel are selected from a preset number of combinations; and aspecified multiplicity, which indicates a number of the informationunits in the control channel to which the same combination is applied,is reported separately for uplink and downlink.
 4. The base station asclaimed in claim 1, wherein for each subframe, combinations ofmodulation schemes and channel coding schemes to be applied toinformation units provided for the respective user devices in thecontrol channel are selected from a preset number of combinations; and aspecified multiplicity, which indicates a number of the informationunits in the control channel in a case where one of the preset number ofthe combinations of modulation schemes and channel coding schemes thatprovides a highest transmission rate is applied to the control channel,is reported separately for uplink and downlink.
 5. The base station asclaimed in claim 1, wherein for each subframe, combinations ofmodulation schemes and channel coding schemes to be applied toinformation units provided for the respective user devices in thecontrol channel are selected from a preset number of combinations; and aspecified multiplicity, which indicates a total number of theinformation units in the control channel in a case where one of thepreset number of the combinations of modulation schemes and channelcoding schemes that provides a highest transmission rate is applied tothe control channel, is reported collectively for uplink and downlink.6. The base station as claimed in claim 1, wherein the control channelis transmitted for each subframe as L1/L2 control information;modulation and coding scheme information indicating a combination of amodulation scheme and a channel coding scheme to be applied to thecontrol channel is transmitted as L3 control information; andinformation indicating a maximum number of information units providedfor the user devices in the control channel is reported separately foruplink and downlink.
 7. The base station as claimed in claim 1, whereinthe control channel is transmitted for each subframe as lower layercontrol information; modulation and coding scheme information indicatinga combination of a modulation scheme and a channel coding scheme to beapplied to the control channel is transmitted as upper layer controlinformation; and information indicating a total number of informationunits provided for the user devices in the control channel is reportedcollectively for uplink and downlink.
 8. The base station as claimed inclaim 1, wherein the control channel is transmitted for each subframe aslower layer control information; modulation and coding schemeinformation indicating a combination of a modulation scheme and achannel coding scheme to be applied to the control channel istransmitted as upper layer control information; information indicating amaximum number of information units provided for the user devices in thecontrol channel to be transmitted in a given subframe is reportedseparately for uplink and downlink via broadcast information;information indicating a total maximum number of the information unitsprovided for the user devices in the control channel to be transmittedin each subframe is reported collectively for uplink and downlink; andrelative positions of uplink control information and downlink controlinformation in the control channel are predetermined.
 9. The basestation as claimed in claim 1, wherein the control channel istransmitted for each subframe as lower layer control information;information indicating a maximum number of information units providedfor the user devices in the control channel to be transmitted in a givensubframe is reported separately for uplink and downlink via broadcastinformation; information indicating a total maximum number of theinformation units provided for the user devices in the control channelto be transmitted in each sub frame is reported collectively for uplinkand downlink; and relative positions of uplink control information anddownlink control information in the control channel are predetermined.10. A transmission method for a mobile communication system employingorthogonal frequency division multiplexing (OFDM) for downlink and usingsubframes each including multiple OFDM symbols, the transmission methodcomprising the steps of: mapping a control channel to a predeterminednumber of OFDM symbols from a beginning of each subframe and mapping adata channel to OFDM symbols that follow the OFDM symbols to which thecontrol channel is mapped; and transmitting the control channel and thedata channel mapped in the mapping step, wherein multiple controlresource blocks are multiplexed in the control channel mapped in themapping step and each of the control resource blocks is mapped to everyone of the OFDM symbols to which the control channel is mapped; and afixed transmission format is used for transmission of the controlchannel to some user devices in a cell and various transmission formatsare used for transmission of the control channel to other user devicesin the cell.
 11. The transmission method as claimed in claim 10, whereinthe control channel includes paging indicator identification informationthat is different from user device identification information.
 12. Amobile communication system employing orthogonal frequency divisionmultiplexing (OFDM) for downlink and using subframes each includingmultiple OFDM symbols, the mobile communication system comprising: abase station configured to transmit a signal via downlink; and userdevices configured to receive the signal from the base station, whereinthe base station includes a mapping unit configured to map a controlchannel to a predetermined number of OFDM symbols from a beginning ofeach subframe and to map a data channel to OFDM symbols that follow theOFDM symbols to which the control channel is mapped; and a transmittingunit configured to transmit the control channel and the data channelmapped by the mapping unit; multiple control resource blocks aremultiplexed in the control channel mapped by the mapping unit and eachof the control resource blocks is mapped to every one of the OFDMsymbols to which the control channel is mapped; and a fixed transmissionformat is used for transmission of the control channel to some of theuser devices in a cell and various transmission formats are used fortransmission of the control channel to the other user devices in thecell.
 13. A mobile communication system employing orthogonal frequencydivision multiplexing (OFDM) for downlink and using subframes eachincluding multiple OFDM symbols, the mobile communication systemcomprising: a base station including a mapping unit configured to map acontrol channel to a predetermined number of OFDM symbols from abeginning of each subframe and to map a data channel to OFDM symbolsthat follow the OFDM symbols to which the control channel is mapped; atransmitting unit configured to transmit the control channel and thedata channel mapped by the mapping unit; user devices each including areceiving unit configured to receive the control channel and the datachannel; and a processing unit configured to process the control channeland the data channel received by the receiving unit, wherein multiplecontrol resource blocks are multiplexed in the control channel mapped bythe mapping unit and each of the control resource blocks is mapped toevery one of the OFDM symbols to which the control channel, is mapped,and wherein a fixed transmission format is used for transmission of thecontrol channel to some of the user devices in a cell and varioustransmission formats are used for transmission of the control channel tothe other user devices in the cell.